LIMITS OF TOXICITY 137 



portant part in their toxicity. In Colorado (17), however, 

 soils containing rather large quantities of nitrates were 

 found to be still active in nitrifying, although when the 

 nitrates became excessive the organisms were destroyed 

 or greatly checked in their activity. 



Kelley (8) found that the nitrite-forming organisms 

 were still active in soil containing so much alkali that 

 nitrate formation had practically ceased. 



Nitrogen-fixing organisms were found by Lipman and 

 Sharp (14) to be inhibited by the presence of 4000 to 5000 

 parts per million of sodium carbonate. The toxic limits 

 for sodium chloride were 5000 to 6000 parts per million, 

 and for sulphate about 12,500 parts per million. Much 

 smaller quantities were found injurious where the soil was 

 leached of its salts, the quantity in this experiment being 

 nearly the same as with the nitrifying bacteria (13). 



Hills (7) reports that 1500 parts per million of sodium 

 nitrate stopped multiplication and probably killed many of 

 the nitrogen-assimilating organisms. Symbiotic bac- 

 teria (15) on peas were retarded in their activities when 

 sodium salts in cultural solutions with a strength of 3333 

 parts per million were used. Alkaline nitrates at a con- 

 centration of 100 parts per million and ammonium salts 

 at a concentration of 500 parts per million checked the 

 production of root tubercles. 



Ammonification organisms have been found by investi- 

 gators who have experimented with them in comparison 

 with those concerned with nitrification and nitrogen- 

 fixation to be more tolerant of alkali than these other 

 nitrogen-working organisms. Lipman found the toxic 

 points for ammonification to be at 20,000 parts per 

 million of sodium carbonate, 1000 to 2000 parts per 

 million of sodium chloride, and 4000 parts per million 



